Ultrasound method and apparatus for examining dense tissues, in particularly dental tissue

ABSTRACT

The invention concerns an ultrasound based measurement method and an apparatus for examining dense tissues in particular. According to the method, the measured object (17) is subjected to a high-frequency ultrasound signal, and echoes returning from the object (17) are converted into electric signals. According to the invention, either the object (17) or the ultrasound transducer (36) is vibrated at a low-frequency and the level of the low-frequency vibrating signal is detected from the echo signal, whereby interface locations of the vibrating object (17) can be determined from the maximum values of the detected low-frequency signal in relation to the transmitted high-frequency pulse.

BACKGROUND OF THE INVENTION

The present invention relates to a measurement method for examiningdense tissue with the help of ultrasound.

The invention also relates to a measurement apparatus for theimplementation of the method.

In conventional methods, the examination of dense tissue is performedusing X-ray machines. Also apparatuses for measuring dense tissue bymeans of ultrasound are known in the art. The publication JOURNAL OFCLINICAL ULTRASOUND (No. 13, October 1985, pages 597...600, article"Development and Application of an Ultrasonic Imaging System for DentalDiagnosis"), describes an ultrasound measurement method for examiningteeth.

Because of the health hazards involved with X-rays, it is impossible toobtain real-time information about the treatment operation; instead, onemust be satisfied with single pictures. Conventionally, the preparationof exposing and developing an X-ray picture takes a relatively longtime, about 7...10 minutes. The ultrasound equipment currently used inmedicine applications further require a computer which must be able toperform relatively complicated computations, and even with a powerfulcomputer it is, e.g., impossible to reliably define the form of the rootchannel of a tooth or the position of a broach needle in the channel.

SUMMARY

The aim of the present invention is to overcome the drawbacks of theaforementioned prior art technology and to provide a completely novelultrasound-based measurement method and apparatus for examining densetissue.

The invention is based on subjecting the dense tissue under examinationto low-frequency vibration. A high-frequency ultrasound transmitted tothe tissue is modulated by the low-frequency vibration. The reflectedultrasound signal is processed by timed gating, and the vibration signalcomponent is band pass filtered for further processing. Interfaces ofthe dense tissue are revealed by the amplitude maxima of the processedsignal.

Consequently, the method in accordance with the invention provides for areal-time localization of tissue interface profiles as well as treatmentinstruments during the treatment operation. The apparatus is easy to useand is harmless to the patient. Furthermore, the apparatus iscost-effective in terms of attained resolution and ease of use.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, detailed description the invention will be set forthin more detail by means of the exemplifying embodiments in accordancewith the attached drawings, wherein

FIG. 1 shows a measurement system in accordance with the invention.

FIG. 2 shows a computer-processed image obtained by means of the systemin accordance with the invention.

FIG. 3 shows in a block diagram the measurement system in accordancewith the invention.

FIG. 4 shows in a block diagram one measurement channel in the systemillustrated in FIG. 3.

FIG. 5 shows a timing diagram of the signals over one vibrating cycle inthe measurement system illustrated in FIG. 4.

FIG. 6 shows in a perspective view another measurement system inaccordance with the invention.

FIG. 7 shows in a partially diagrammatic top view the operation of thesystem illustrated in FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

Using a laboratory setup illustrated in FIG. 1, a tooth underexamination was scanned by means of an X-Y table 1 under a transducer 3mounted to a drill arm 2. The X-Y table was controlled by a computer 4via stepper motors 5 attached to the table's micrometer screws. Inaddition, between the stepper motors 5 and the computer 4, there wasattached a separate controller 6, which served as a buffer amplifier forthe stepper motors 5. The transducer 3 was a focused 5 MHz ultrasoundtransducer. Normally, the ultrasound transducer 3 comprises both atransmitter and a receiver section. Currently, most transducers are madeof a ceramic piezoelectric material. A needle probe 7 was attached tothe metal foil of a miniature buzzer 8 with the help of contact cement.The miniature buzzer 8 was driven by an oscillator 9, which had anoscillating frequency adjustable in the range of 200...600 Hz. The inputpower of the oscillator 9 was about 4 W, however, the produced outputintensity level remained appreciably lower than in commerciallyavailable vibrators. A conventional laboratory power supply was used asa power supply 10. Required energy pulses to the transducer weregenerated by a pulser 11, which also served as a receiver and amplifierfor the received echo signals. The received echo pulse was gated by agating unit 12 with such a timing as to allow only the echo from thearea under examination to pass through the gate. A filter 13 was usedfor extraction of the vibration frequency of the needle 7 by band passfiltering from the echo pulse, and a rectifier 14 provided the vibrationlevel signal by rectification. The detected signal level was measured bya voltage meter 15, integrated to the system and controlled by thecomputer 4. The waveforms were monitored by an oscilloscope 16. Inaddition, the computer 4 supervised the entire measurement session andcollected measurement data onto a diskette. The sweep pattern over thetooth under examination was organized in a 25 by 40 cell matrix, inwhich the cell size was 0.5×0.5 mm². The recorded voltage levels werestored on diskettes. Each measurement session over one examination areatook about 45 minutes. The examined object was a swine jawbone.

The measurements were performed using the following basic settings: Thedistance of the transducer 3 from the needle probe 7 was approx. 20 mm,the transducer drive signal frequency was 5 MHz, the delay of the needleecho in water was approx. 25 us, and the needle vibrating frequency wasapprox. 500 Hz.

The tooth under examination was immersed in water, and the rear teeth(molars) were selected for examination. The bath temperature was approx.20° C. Data stored on a diskette was analyzed using a BASIC-languageprogram. Input data for the program was taken from the high levels ofsignal voltage values caused by the needle vibration as well as thevoltage levels caused by the tooth's vibration where detectable from thenoise floor. Picture generation onto the screen took about one minute.The slowness of program execution was mainly related to the programminglanguage used. Successful results in examinations were obviously alsoattained by wetting the tooth under examination and placing a rubberbladder between the tooth and the transducer.

Illustrated in FIG. 2 is the measured matrix in approx. 12-foldenlargement. Thus, the matrix cell in the figure has dimensions ofapprox. 6×6 mm². The matrix represents one longitudinally sectionedslice of the tooth. Each matrix cell corresponds to one measurementresult, which in practice is related to one measured DC voltage value.The needle probe 7 is visible in the middle of the figure as ahorizontal bar-like area with the darkest rasterization. The jawbone isto be found at the right side of the figure. Thus, the location of theneedle probe is easily definable with the help of the arrangement inaccordance with the invention. Further, the identification of the toothprofile against the background is possible by averaging severalsubsequently taken pictures. The illustration is enhanced with borderlines that define the pulp cavity 30 and the outer surface of the tooth17.

The quality of measurement results was not essentially influenced by theoperating frequency of the modulating needle. By contrast, the needleoperating frequency had a significant effect on the measurement speed. Ahigher frequency resulted in a correspondingly faster measurement.Additionally, the disturbance level was related to the needle operatingfrequency. Namely, problems arose from the difficulty of finding such avibrating frequency, at which the interaction of the pulser's pulse rate(approx. 5.5 kHz) with the power line (50 Hz) disturbances would be at aminimum. A proper frequency range was found at 200...800 Hz. Incommercial applications the pulse rate should be increased to about 20kHz so that the needle vibrating frequency could be about 1 kHz. In thismanner, the measurement speed would be sufficiently fast, andfurthermore, filtering of the useful signal would be easy to realize.

According to FIG. 3, a commercially produced measurement apparatuscomprises a transducer unit 18 and an analyzer unit 19, which furthercomprises an analog section 20, a data retrieval section 21, a dataprocessing section 22, and a picture processing section 23. In addition,the system includes a vibrator (not shown), which is a compressed-airoperated pulp treatment device conventionally used by dentists. Thetransducer unit 18 includes a linear array ultrasound transducer as wellas an actuator mechanism for moving the transducer. This arrangementreplaces the X-Y table used in the system during laboratory experiments.The transducer unit 18 also includes the electronic circuits requiredfor amplification of transducer signals and control of the actuatormechanism. The design of the transducer unit 18 resembles an electrictoothbrush, which is pushed against the tooth during the examination.The transducer unit 18 is cabled to the analyzer unit 19. The purpose ofthe data retrieval section 21 is to control the measurement and togather measurement results from the analog section 20 to the RAM memory.The data processing section 22 fetches measurement results from thememory of the data retrieval section 21 and performs required operationsfor pattern recognition. The picture processing section 23 generates theimage files and controls the CRT or other similar output device. Thesystem is capable of achieving a picture generation and update rate ofat least 1 picture/10 s.

Illustrated in FIG. 4 is one measurement channel of the analyzer unit 19serving one transducer. The total number of channels equals the numberof transducers in the linear array. The data retrieval section 21controls the operation of the actual transducer unit 18 and the analogsection 20. The data retrieval section 21 gives required controlinformation to the timer 22 and controls the gain of the preamplifier A₂as well as the length of windowing delays. The timer 22 delivers controlpulses U₀ to pulser 24 and issues the start moment of the windowingdelay to a delay section 25. The programmable delay section 25 generatesa required control signal U₈ of the gating window to an analog switch26. The pulser 24 delivers control pulses U₁ to the transducer 27 at alevel matched for the ultrasound transducer. The ultrasound transducer27 is of the pulse echo type. The amplifier A₁ amplifies the echo signalU₁ by approx. 20 dB in order to obtain a sufficiently high signal U₂ tobe transferred by the cable to the analyzer unit 19. Voltage level of anecho signal U₂ amplified by a controllable amplifier A₂ is adjusted to aproper level for AM detection. An AM detector 28 generates a signal U₄which is the envelope signal of an amplified echo signal U₃. An analogswitch 26 gates for further processing an echo signal U₅, which is theecho received from the desired depth to be examined. A bandpass filter29 is used to separate a modulating vibrating frequency U₆ from the echosignal U₅. An amplitude signal U₇ of the vibrating frequency is detectedby rectification from the filtered vibration frequency signal U₅ by arectifier 31. The rectified signal U₇ is converted by an A/D converter32 into an appropriate format for digital processing.

Illustrated in FIG. 5 are the waveforms over one vibrating cycle atdifferent points of the exemplifying block diagram shown FIG. 4. Priorto the illustrated cycle, a sufficiently high number of vibrating cyclesare measured so that the voltage level of signal U₇ has already receivedits final value. One division of the diagram corresponds to approx. 10us so that the modulation frequency is about 2 kHz.

In accordance with FIG. 6, a needle 7 of a root channel broach 33 isinserted into the pulp of a tooth 17. The broach needle 33 is operatedby compressed air, which is led to the broach instrument 33 via hoses34. The ultrasound transducer unit 35 proper consists of a linear arraytransducer 36, which comprises of, e.g., 8 separate transducers. Thelinear array transducer 36 is mounted within an array body 37 whosedimensions are: depth a approx. 16 mm, length b approx. 20mm, and heightc approx. 20 mm. The array transducer 36 is movable within the body 37about its longitudinal axis with the help of a motor 39 mounted tosupporting arm of the ultrasound transducer unit 35 and an actuatormechanism 38 mounted integral with the transducer body 37. Between thetransducer body 37 and tooth 17 is inserted a bladder 42, filled with,e.g., water. Close to the actuator motor 39 in the handle part of theultrasound unit 35 is mounted an electronics unit 40, which serves forthe control of the motor 39 and reprocessing of signals. The measurementsignals are routed via a cable 41 for further processing.

In accordance with FIG. 7 the thickness w of the detected tissue sliceis determined by the size of the gating window. With a narrower window,a thinner slice is analyzed, which consequently offers an improved depthresolution by narrowing the gating window. Hence, the depth location ofthe slice is determined by the timed shifting of the window.Consequently, the further the gating window is shifted on the time scalefrom the send instant of the pulse, the deeper the slice underexamination is shifted along the depth axis. By virtue of the modulationgenerated by the vibration of the needle 7, the echo signals obtainedfrom the array transducer 36 that is i.e., the echoes received from theneedle and other vibrated objects present a signal level significantlyhigher than any other signals received from other areas within the gatedsignal window.

The operating parameters of an apparatus according to the invention maybe varied as follows: vibrator's vibrating frequency approx. 100...1000Hz, appropriate ultrasound frequency approx. 3...15 MHz, and echo signalamplification approx. 20...60 dB.

The method in accordance with the invention can be used in addition tothe medical applications, also for examining such physical compositestructures, in which hard material is combined with pliabe material.

Replacing the vibration of the measured object, the transducer itselfmay alternatively be vibrated at a low frequency. An essentialcharacteristic of an embodiment in accordance with the invention is therelative movement between the measured object and the transducer at alow frequency.

What is claimed is:
 1. An ultrasonic method for examining an ultrasonicmedium including dense tissues, said method being repetitively performedat periodic intervals, comprising the steps of:generating ahigh-frequency ultrasonic pulse by means of an ultrasound source (36),transmitting the ultrasonic pulse to an object (17) under examination,converting echo signals returning from the object (17) into electricalsignals, determining the intensity and delay of returning echo signalsin relation to the pulse transmitted to the object under examination,generating an image of the measured object (17) under examination fromthe determined delay and intensity, vibrating the measured object (17)or the ultrasound source (36) at a low frequency, and wherein theconverting step further includes detecting and extracting a signal levelof the low-frequency vibration signal from the echo signal in order toreveal the location of interfaces of the vibrating object(17) from thedelays of the maximum values of the detected low-frequency signalrelative to the transmitted high-frequency ultrasonic pulse.
 2. A methodin accordance with claim 1 wherein said step of vibrating comprisesvibrating the measured object (17) at a frequency in the range of
 100. ..1000 Hz.
 3. A method in accordance with claim 2 wherein the object (17)under examination comprises a tooth and said step of vibrating includesinserting a compressed air operated needle (7) into a root channel (30)of the tooth.
 4. A method in accordance with claim 2 wherein the step ofdetecting and extracting the signal level of the low-frequency vibratingsignal comprises detecting the echo signal, gating the detected echosignal, filtering the gated echo signal, and rectifying the filteredsignal, whereby the interface locations of the vibrating object 17 canbe determined from the time relationship of the maximum value of therectified signal to the transmitted ultrasonic pulse.
 5. Ultrasonicmeasurement apparatus for examining an ultrasonic medium including densetissues, comprising:an ultrasonic pulse generator (24), for generating adrive signal, an ultrasonic transducer (36) connected to the pulsegenerator (24), said transducer converting the drive signal into anultrasonic pulse transmitted towards an object (17) under examination,an ultrasonic receiver (36) for converting an ultrasonic echo pulsereflected from said object into an electric signal, and a measurementand analyzer system (19) coupled to the receiver (36) for determiningthe intensity and delay of a vibration signal received by the receiver(36) in relation to the the ultrasonic pulse transmitted thus making itpossible to generate a model of said object under examination, saidsystem further comprising, a vibrator (33) for vibrating said object(17) or the ultrasound transmitter (36) with a relatively low frequencysignal, and detection apparatus (20) coupled to said receiver (36), fordetecting the level of the low-frequency vibrator signal from the echosignal for determining the interface locations of said object (17) fromthe maximum values of the low-frequency vibrator signal in relation tothe drive signal.
 6. An apparatus in accordance with claim 5, whereinsaid vibrator (33) comprises a compressed-air-operated needle (7)inserted into a root channel of a tooth (17) under examination.
 7. Anapparatus in accordance with claim 5 or 6, wherein said detectingapparatus (20) includes, an AM detector (28), for detecting thelow-frequency vibrator signal in the echo pulse; a signal gate (26, 25)for gating out the detected vibrator signal,and a rectifier (31), forrectifying the filtered signal, whereby interface locations of thevibrating object, at least, at least, can be determined from the timerelationship of the maximum value of the rectified signal to theultrasonic pulse transmitted.
 8. An apparatus in accordance with claim7, wherein the filter (29) comprises a bandpass filter having a centerfrequency tuned to the frequency of said vibrator.
 9. A method inaccordance with claim 1 wherein said step of generating a high-frequencyultrasonic pulse comprises generating a pulse having a frequency in therange of
 3. . .15MHz and said step of vibrating comprises vibrating theobject under examination at a frequency in the range of
 100. . .1000Hz.10. An ultrasonic method for examining an ultrasonic medium includingdense tissues, said method being repetitively performed at periodicintervals, comprising the steps of:generating a high-frequencyultrasonic pulse by means of an ultrasonic source (36), transmitting theultrasonic pulse to an object (17) under examination, converting echosignals returning from the object (17) into electric signals,determining the intensity and delay of returning echo signals inrelation to the pulse transmitted to the object under examination,generating an image of the measured object (17) under examination fromthe determined delay and intensity, vibrating the measured object (17)or the ultrasound source (36) at a low frequency, wherein the convertingstep further includes detecting and extracting a signal level of thelow-frequency vibration signal from the echo signal in order to revealthe location of interfaces of the vibrating object (17) from the delaysof the maximum values of the detected low-frequency signal relative tothe transmitted high-frequency ultrasonic pulse; and wherein said objectunder examination comprises a tooth and said step of vibrating furtherincludes inserting a vibrating needle into a root channel (30) of thetooth.
 11. Ultrasonic measurement apparatus for examining an ultrasonicmedium including dense tissues, comprising:an ultrasonic pulse generator(24), for generating a drive signal, an ultrasonic transducer (36)connected to the pulse generator (24), said transducer converting thedrive signal into an ultrasonic pulse transmitted towards an object (17)under examination, an ultrasonic receiver (36) for converting anultrasonic echo pulse reflected from said object into an electricsignal, and a measurement and analyzer system (19) coupled to thereceiver (36) for determining the intensity and delay of a vibrationsignal received by the receiver (36) in relation to the ultrasonic pulsetransmitted for generating a model of said object under examination,said system further comprising, a vibrator (33) for vibrating saidobject (17) or the ultrasound transmitter (36) with relatively lowfrequency signal, detection apparatus (20) coupled to said receiver(36), for detecting the level of the low-frequency vibrator signal fromthe echo signal for determining the interface locations of said object(17) from the maximum values of the low-frequency vibrator signal inrelation to the drive signal, and wherein said vibrator (33) comprises acompressed-air-operated needle (70) inserted into a root channel of atooth (17) under examination.